The present invention relates to a spread spectrum receive apparatus used for digital cellular mobile communication and the like.
A spread spectrum communication scheme is used as one of the transmission schemes in digital mobile communication using portable telephones and the like. According to the CDMA (Code Division Multiple Access) scheme as a typical scheme of the spread spectrum communication schemes, transmission data is spread-spectrum-modulated with quadrature code sequences to be transmitted, and the data is despread at the receiving end with code sequences identical to those used for spread spectrum modulation so as to extract the original data. This scheme allows multiplexing in code areas by providing different code sequences to different users, and has the advantage of high interference resistance.
As a receive apparatus used in such a spread spectrum communication scheme, and particularly the CDMA scheme, an arrangement using a RAKE receiver like the one shown in FIG. 1 is known. In this receive apparatus, a signal received by an antenna 41 is subjected amplification, frequency conversion, and the like in a RF receive circuit 42. The resultant signal is converted into a digital signal by an A/D converter 43. This signal is then input to a RAKE receiver 44.
In the RAKE receiver 44, a correlation process section 45 first performs a correlation process for the signal with a known code sequence to estimate a plurality of times at which the maximum correlation on the time axis is provided. A despreading process section 46 then performs a despreading process (a cross-correlation process between the received signal and the known code sequence) for the received signal at each of these maximum correlation times. Subsequently, a combine section 47 performs time correction for the respective output signals from the despreading process section 46 to make them in phase, and combines them to output a demodulated signal.
With the above processing in the RAKE receiver 44, a plurality of delay waves (multipath waves) received by the antenna 41 with different delays can be combined, thus increasing the SNR (signal-to-noise ratio) of the demodulated signal. The demodulated signal output from the RAKE receiver 44 is decoded by a decoder 48. Even in the presence of multipath waves, the original data can be properly regenerated. The RAKE receiver operates very effectively as a receiver for mobile communication in the presence of many multipath waves.
Although a spread spectrum receive apparatus using this conventional RAKE receiver can effectively combine a plurality of multipath waves with different delays, this apparatus cannot properly cope with changes in incoming polarized waves. More specifically, in the actual radio wave propagation environment for mobile communication, the polarized waves of transmitted signals generally change every moment owing to the manners in which users use their terminals and reflection and diffraction on radio wave propagation paths. In many cases, a plurality of multipath waves arriving at the receive apparatus differ in main polarized components. The spread spectrum receive apparatus in FIG. 1 can basically receive only one polarized component, and hence cannot combine a plurality of multipath waves having different main polarized components. Therefore, an increase in SNR of a demodulated signal with the use of a RAKE receiver cannot be expected.
As described above, the spread spectrum receive apparatus using the conventional RAKE receiver cannot properly cope with incessant changes in the polarized waves of incoming waves, and hence cannot combine a plurality of multipath waves having different main polarized components. If polarization diversity is to be used to solve this problem, a plurality of RAKE receivers corresponding to a plurality of polarized components must be prepared, resulting in a complicated, expensive apparatus.
The present invention has been made to solve the above problems, and has as its object to provide a spread spectrum receive apparatus which can realize an increase in SNR with a simple receiver arrangement by combining all received signals even if a plurality of radio waves with different delays and different polarized components are received.
In order to achieve the above object, a spread spectrum receive apparatus according to the present invention comprises a plurality of receive branches, wherein each of the plurality of receive branches includes at least one antenna which receives a plurality of different polarized components; and a RAKE receiver, commonly prepared for the plurality of receive branches, for modulating the plurality of different polarized components received, and outputting a modulated signal based on the plurality of different polarized components modulated.
Since a plurality of different polarized components are received and can be combined by the RAKE receiver, even if a plurality of multipath waves with different polarized components as well as different delays arrive, the multipath waves can be combined. An increase in SNR can therefore be attained by using the RAKE receiver.
More specifically, the RAKE receiver comprises a plurality of correlation process means for performing correlation processes between the respective received signals from the plurality of receive branches and a known code sequence, and estimating a plurality of times at which a maximum correlation is provided on a time axis, a plurality of despreading process means for performing a despreading process for the received signals at the plurality of times estimated by the correlation process means, and combine means, commonly prepared for the received signals from the plurality of receive branches, for performing time correction for output signals from the despreading process means to make the signals in phase, and combining the signals to output a demodulated signal.
With this arrangement, one RAKE receiver can be commonly used for all the receive branches, and there is no need to prepare a plurality of receive systems each including a RAKE receiver as in a case wherein polarization diversity is to be performed. The overall arrangement of the receive apparatus can therefore be greatly simplified.
In addition, this apparatus may include a delay means for setting a relative delay between received signals from the plurality of receive branches. With this arrangement, even if the polarized components received by the respective receive branches have almost the same delay profile, the need for high-precision delay estimation in the RAKE receiver can be eliminated by setting a large delay difference between the polarized components. This facilitates the processing in the RAKE receiver.
Another spread spectrum receive apparatus according to the present invention comprises a plurality of receive branches including an antenna for receiving a plurality of different polarized components, delay means for providing relative delays to received signals from the plurality of receive branches, combine means for combining the received signals from the respective receive branches through the delay means, and a RAKE receiver for receiving a combined received signal output from the combine means and outputting a demodulated signal.
With this arrangement, one RAKE receiver can be commonly used for received signals from the plurality of receive branches to combine the signals. This further simplifies the overall arrangement of the receive apparatus.
In this case, the RAKE receiver comprises correlation process means for performing correlation processes between the combined received signal from the combine means and a known code sequence, and estimating a plurality of times at which a maximum correlation is provided on a time axis, a plurality of despreading process means for performing despreading processes for the combined received signal at the plurality of times estimated by the correlation process means, and combine means for performing time correction for output signals from the despreading process means to make the signals in phase, and combining the signals to output a demodulated signal.
As an antenna for receiving a plurality of different polarized components in the present invention, an antenna may be independently prepared for each polarized component. However, for example, a rectangular patch antenna can be used such that two quadrature linear polarized components or two quadrature circular polarized components are received by this physically single antenna and are separately output.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.